The present invention relates, in general, to semiconductor device manufacturing process and, in particular, to coating of substrate containing semiconductor region or of semiconductor substrate.
Light is produced by the transition of electrons from higher energy states to lower energy states. The law of conservation of energy is satisfied in these transition processes by the emission of a photon, or quantum of light, whose energy corresponds to the difference in energy of the initial and final energy states of the electron. In nature, photons are created spontaneously at many different frequencies. In Light Amplification by Stimulated Emission of Radiation (laser), a light wave having a wavelength that corresponds to the difference between a high energy state and a low energy state that strikes an electron in the high energy state will cause the electron to transition to the low energy state and emit a photon with the same direction, phase, polarization, and frequency as the incident photon of the light wave. Thus, a traveling light wave of a certain frequency is produced.
A semiconductor ridge laser is useful as an element in a photonic integrated circuit because it emits light horizontally which can be processed by another element which is formed on the horizontal plane of the substrate of the photonic integrated circuit. A ridge laser that includes an active layer in which a traveling light wave is stimulated has been produced by many different means.
A dielectric layer is formed on each end, or facet, of an optical device to increase or decrease the reflectance of each facet. Typically, the facets are cleaved. The reflectance of a facet without a dielectric layer deposited thereon is a function of the difference in refractive index of the optical device material and the transmission medium (e.g., air). The reflectance of a facet with a dielectric layer deposited thereon is a function of the difference in refractive indexes of the optical device material, the dielectric layer material, the transmission medium, and the thickness of the dielectric layer.
A dielectric layer may be deposited onto a facet (i.e., a coated facet) to either increase or decrease the reflectance of the facet coated with a dielectric layer as compared to the facet without a dielectric layer deposited thereon (i.e., an uncoated facet). Increasing facet reflectance is one method of reducing the minimum cavity length at which a semiconductor laser will operate. Minimizing the cavity length of a semiconductor laser allows for increased packing density, increased operating speed, lowered operating current, and reduced power consumption. Decreasing facet reflectance prevents lasing of an optical device and allows the resulting device to operate as an optical amplifier.
To reduce the cavity length of a semiconductor laser, facet reflectance must be increased. To increase the reflectance of a facet, at least one pair of dielectric layers must be deposited on the facet, where the optical thickness of each dielectric layer within the pair is an odd integer multiple of quarter wavelengths, and where the first dielectric layer in a pair has a lower index of refraction than the second dielectric layer in the pair. The optimal thickness is t=mxcex/n4, where m is an odd integer, where xcex is the free-space emission wavelength of the laser, and where n is the refractive index of the dielectric layer.
To form an optical amplifier, facet reflectance must be reduced to prevent lasing. To decrease the reflectance of a facet, at least one single dielectric layer must be deposited on the facet, where the optical thickness of the dielectric layer is an odd integer multiple of quarter wavelengths. The optimal thickness is t=mxcex/n4, where m is an odd integer, where xcex is the free-space emission wavelength of the optical amplifier, and where n is the refractive index of the dielectric layer. The optimal refractive index of the dielectric layer is equal to the square root of the refractive index of the optical device for minimal reflectance.
One prior art method of increasing or decreasing facet reflectance is by thermally evaporating a dielectric material in one direction (e.g., up) onto the facet. With such a method, thermal evaporation must be done a first time at a first orientation of an optical device and then a second time at a second orientation (i.e., rotated 180 degrees). Since unidirectional thermal evaporation is slow and requires precise physical manipulation of the device, it is not a suitable process for a conventional microelectronics process in which a substantial number of devices are manufactured at the same time. Unidirectional thermal evaporation is mentioned in a first article entitled xe2x80x9cFacet-Coating Effects on the 1.3 xcexcm Strained Multiple-Quantum-Well AlGaInAs/InP Laser Diodes,xe2x80x9d by Chia-Chien Lin et al., published in the Japanese Journal of Applied Physics, Vol. 37 (1998), pp. 6399-6402 and in a second article entitled xe2x80x9cInGaAs/InP quantum well lasers with sub-mA threshold current,xe2x80x9d by H. Temkin et al., published in the Applied Physics Letters, Vol. 57, No. 16, Oct. 15, 1990.
Another method of increasing or decreasing facet reflectance is by etching at least one air gap in a semiconductor material near the facet. The air gap may be filled with a dielectric material to reduce diffraction losses. Etching and filling air gaps are additional steps that take time and are prone to error.
A semiconductor Bragg reflector composed of air gaps and semiconductor layers may be etched to increase or decrease the reflectance of a facet. Etching a Bragg reflector requires an electron-beam (i.e., e-beam) lithography machine that is expensive and slow. E-beam machines have sufficient resolution to define etched layers to form a high order Bragg reflector, but do not have sufficient resolution to define etched layers required to form a first order Bragg reflector and, therefore, cannot maximize optical reflection efficiency. An E-beam process is disclosed in a first article entitled xe2x80x9cEdge-Emitting Lasers with Short-Period Semiconductor/Air Distributed Bragg Reflector Mirrors,xe2x80x9d by Y. Yuan et al., published in the IEEE Photonics Thechnology Letters, Vol. 9, No. 7, pp. 881-883, July 1997, a second article entitled xe2x80x9cEdge-Emitting GaInAs-AlGaAs Microlasers,xe2x80x9d by E. Hofling et al., published in IEEE Photonics Technology Letters, Vol. 11, No. 8, pp. 943-945, August 1999, and a third article entitled xe2x80x9cContinuous Wave Operation of 1.55 xcexcm GaInAsP/InP Laser with Semiconductor/Benzocyclobutene Distributed Bragg Reflector,xe2x80x9d by Mothi Madhan et al., published in the Japanese Journal of Applied Physics, Vol. 38, pp. L1240-L1242, Nov. 1, 1999. The third article provides details on filling air gaps with a dielectric.
A vertical cavity surface emitting laser (VCSEL) is a laser in which mirrors are formed by depositing epitaxial layers. Epitaxial layer deposition is a method that is compatible with microelectronics fabrication techniques, but a VCSEL emits light vertically and, therefore, is not useful as an element in a photonic integrated circuit which requires horizontal light emission.
It is an object of the present invention to fabricate an optical device in which light travels horizontal to a substrate on which the optical device is formed so that the optical device is useful as an element in a photonic integrated circuit.
It is another object of the present invention to modify the reflectance of facets of an optical device in a conformal coating manner that is compatible with conventional microelectronics processes.
The present invention is a method of simultaneously depositing dielectric material onto multiple facets of an optical device, where the facets are in multiple orientations, to change the reflectance of the facets using a conformal coating process.
The first step of the method is selecting a substrate that supports optical activity.
The second step of the method is forming an optical device on the substrate.
The third step of the method is forming at least one active-layer pump structure for operating the optical device.
The fourth step of the method is forming a facet on each end of the optical device.
The fifth step of the method is simultaneously coating a user-definable number of dielectric layers on the facets of the optical device, where the facets have at least two different orientations.
Various devices such as a laser and an optical amplifier may be made using the method of the present invention.